A 52-year-old man with a 10-year history of hypertension is found on screening to have a left adrenal nodule (2.2 cm) on CT abdomen done for renal artery stenosis evaluation. His BP is 148/92 mmHg despite triple antihypertensive therapy. Laboratory investigations show: morning cortisol 18 µg/dL (normal 10–20), 24-hour urinary free cortisol 95 µg/24 h (normal <50), plasma ACTH 8 pg/mL (normal 10–50), and potassium 3.2 mEq/L. What is the most appropriate next step in management?